  html{
            height: 100%;
        }
        body{
            height: 100%;
            margin: 0px;
            background:radial-gradient(#383838, #070707);
            display:flex;
            justify-content: center;
            align-items: center;
        }
        main{
            width: 800px;
            height: 500px;
            border: 20px solid #333333;
            border-top: 35px solid #333333;
            border-bottom: 15px solid #333333;
            box-shadow: 0 0 10px gray;
            border-radius: 10px;
            background-image: url("钢琴.png");
            background-size: 100% 100%;
            display: flex;

        }
        div{
            flex-grow: 1;
        }
        main div:active{
            background: linear-gradient(rgba(255, 255, 255, 0),rgba(255, 255, 255, 0),rgba(130, 130, 130, 0.6));
        }